FIG. 1 schematically illustrates a p-type substrate solar cell as one example of prior art solar cells which are generally manufactured on a mass-scale using single crystal and polycrystalline silicon substrates. A pn junction 103 is formed by diffusing Group V element such as phosphorus into the light-receiving surface of a semiconductor substrate (silicon substrate) 101 in a high concentration to form a n-type layer 102. Dielectric films 104 and 105 having a lower refractive index than silicon are formed on both the major surfaces (light-receiving and non-light-receiving surfaces) of p or n-type silicon substrate, respectively, for more efficient containment of light. In these dielectric films 104 and 105, titanium oxide, silicon nitride, silicon carbide, silicon oxide, tin oxide and the like are widely used. While the thickness of a dielectric film that provides for effective optical confinement varies with its refractive index, the thickness of a silicon nitride film, for example, is generally about 80 to 100 nm on the light-receiving surface and about 90 to 300 nm on the back surface.
Also, on the light-receiving surface and the non-light-receiving (back) surface, electrodes 106 and 107 are formed for extracting photo-created carriers. Among methods of forming such electrodes, one method which is widely used from the aspect of cost is by mixing metal fine particles such as silver or aluminum with an organic binder, printing the metal paste using a screen or the like, and heat treating the paste for bringing it in contact with the substrate. Electrode formation is generally preceded by formation of dielectric film. Thus, in order that the electrode make electrical contact with the silicon substrate, the dielectric film between the electrode and the silicon substrate must be removed. This is enabled by tailoring a glass component or additives in a metal paste so that the metal paste may penetrate through the dielectric films 104, 105 to make contact with the silicon substrate, known as the “fire-through” capability.
Another important function of dielectric films 104, 105 is to restrain carrier recombination on the silicon substrate surface. Silicon atoms within crystal are in a stable state due to a covalent bond between adjoining atoms. However, at the surface corresponding to the terminus of an atom array, an unstable energy level, also referred to as unsatisfied valence or dangling bond, develops because an adjoining atom to be bonded is not available. The dangling bond is electrically active enough to capture an electric charge photo-created within silicon whereby the charge is extinguished, thus detracting from the performance of solar cells. To suppress the performance loss, the solar cell is subjected to a certain surface terminating treatment for reducing the dangling bond. Alternatively, the antireflection coating is given electric charges for substantially reducing the concentration of electrons or holes at the surface for thereby restraining recombination of electrons with holes. In particular, the latter is referred to as “field effect passivation.” Silicon nitride and analogous films are known to have positive charges and thus exert the field effect passivation.
However, it is known that if a silicon nitride or analogous film having positive charges is applied to the surface of p-type silicon substrate, solar cell performance is degraded. The positive charge in the film biases the energy band at the p-type silicon surface toward the inverted state, and the concentration of electrons or minority carriers becomes higher at the silicon surface. If an electrode is formed on the p-type silicon surface, then the electrons accumulating on the surface flow to the electrode. Since it is the electrode on the n-type silicon side that extracts electrons in the solar cell, the electrons flowing into the p-type silicon side electrode are lost as leak current flow from the solar cell output. For this reason, a silicon oxide film which allegedly has a relatively low positive charge and an aluminum oxide film having a negative charge are now used for the passivation of p-type silicon surface.
The following technical documents are considered to be relevant to the present invention.